Almost a quarter of care home workers in Northern Ireland have not yet received their jab.
The commissioner for older people Eddie Lynch has urged staff to come forward to receive the vaccine.
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In England it will become compulsory for care home staff in November.
Patricia McKeown, regional secretary for Unison said she is opposed to the same approach being taken in Northern Ireland.
She also urged against the “scapegoating” of care home staff, who she described as having been “so courageous” during the pandemic.
“We have been as a union working with the system to encourage everybody, not just health workers, to take up the vaccination and we will continue to do that,” she told the BBC.
“But we need to look very carefully at what is happening, is it happening in all care homes, is it happening in some, we need that quality information coming from the system, and we’ve been asking for that over the last few days.
“We have some of the highest vaccination rates in Northern Ireland inside some of our care homes so what we have to be clear about is what is causing this.
“We know there has been apprehension from women of a particular age group in terms of pregnancy issues, we know that they got certain information at one stage from the system and now they have more information and different information from the system.
“Let’s find out how those fears can be allayed.
“Unison does not support mandatory vaccination and the evidence, the studies done so far, have indicated that you when you make it mandatory you actually get a negative reaction, more people refuse to be vaccinated than when it is voluntary.
“It could well end up in litigation … you wouldn’t pick out one section of the entire workforce in Northern Ireland and say, ‘it’s you, you’ve got to do this’.
“What you need to continue to do is what we’re trying to do alongside the health system which is convince and persuade people, and where they have fears and doubts, get the right information to them.
“We have written to the Health Minister (Robin Swann) urging him not to follow that (England’s) lead but instead to work co-operatively, the health system, the employers in the sector, unions, the workforce together promoting vaccination but also finding out what the problems are.”
Meanwhile on Friday the jab became available to young people aged 16 and 17.
Walk-in Pfizer first jabs were provided at regional vaccination centres as well as pop-up vaccination clinics.
An online booking platform will be available for this age group in the coming days, the department said.
Northern Ireland was the first part of the UK to give the jab to people in this age group with no underlying medical conditions following the recommendation from the Joint Committee on Vaccination and Immunisation (JCVI) earlier in the week.
Meanwhile the Belfast Health Trust is offering a tickets to see the Cup Final at Windsor Park for those receiving the vaccination.
The trust said it had four pairs of tickets to give away, and everyone who receives a jab at one of four vaccination clinics across the city this weekend will automatically be entered for the draw.